Temporal Decorrelation of GPS Satellite Signals due to Multiple Scattering from Ionospheric Irregularities

Rapid fluctuations in the amplitude and phase of the GPS satellite signals caused by scattering from ionospheric irregularities can degrade GPS performance in a number of ways. These signal fluctuations are referred to as scintillations. Severe scintillations may result in loss of lock, and even when lock is maintained, they may cause errors decoding the GPS data messages and corrupt estimation of the ranges to the GPS satellites. Several previous attempts to model the effects of ionospheric scattering on GPS performance have focused on the depth of signal fading, as quantified by the S4 index, as the appropriate parameterization to use for the ionospheric perturbation of the signal. The S4 index may not be the best parameter for this purpose, however, because it is limited in several respects. Firstly, the S4 index is independent of the rate of signal fading which has a direct impact on the stress of the GPS tracking loops. Secondly, under strong scattering conditions the S4 index saturates to a value near unity, irrespective of the strength of the ionospheric perturbation. On the other hand, the decorrelation time characterizes the rate of signal fading and continues to vary with the strength of the ionospheric perturbation once when the S4 index has saturated. In this paper we characterize the temporal decorrelation of the GPS satellite signals using 20 Hz observations collected at Ascension Island (7.96oS, 14.41oW, dip latitude 12.4oS) on 5-19 March 2002, during solar maximum conditions. We explore the relationships between the ionospheric scintillation parameters, loss of lock, and signal reacquisition time and provide empirical fits to the probability of losing lock for use in modeling and simulation studies. Our results suggest that rate of signal fluctuation, in addition to the depth of signal fading, should be considered when modeling GPS receiver performance in the presence of scintillation. We observed that the probability of loss of lock and likelihood of extended signal reacquisition times depend on the velocity of satellite motion with respect to the magnetic field and plasma drift, which are relatively predictable a priori in the equatorial region.